The default MTU for a T1/E1 was usually 1500. The default for HSSI was 4470 which meant the default for DS3 circuits was 4470. This was also the usual default MTU for IP over ATM which is what most OC-3 circuits would have been using when they were initially rolled out for backbone use. This remained the usual default MTU all the way through OC-192 circuits running packet over sonnet.

I left the lSP backbone and large enterprise WAN field around that time and can't speak to more recent technologies.